The present invention relates to a method for coating a non-oxidised stainless steel support plate with an electrically conducting corrosion-resistant coating, comprising applying a diffusion barrier layer containing a titanium compound, followed by applying a nickel layer. A method of this type is disclosed in German Offenlegungsschrift 19523637. High demands are imposed on that part of the fuel cell which is located on the anode side. On the one hand this must be capable of discharging the stream supplied via the gas distribution device on the anode side. On the other hand this must be sufficiently corrosion-resistant to meet current requirements in respect of service life. Currently a service life of a few ten thousand hours is required. Because of the aggressive environment resulting from carbonate material, high temperature and the relatively low potential at the anode, this side of the separator plate is particularly severely stressed.
In order to avoid the corrosion problems it is proposed in the abovementioned German Offenlegungsschrift to apply a coating, consisting of a titanium nitride layer on top of which a nickel layer has been applied, on the anode side of the stainless steel separator plate. This nickel layer provides protection but the base material from the stainless steel must be prevented from diffusing into the nickel. After all, it has been observed that the strength of the residual nickel layer decreases appreciably as a result of such a diffusion process and within 10 000 hours the residual nickel layer detaches from the stainless steel layer and the cell rapidly becomes inoperative. According to German Offenlegungsschrift 19523637, the thickness of the titanium nitride layer is preferably between 0.5 and 5 xcexcm. It is assumed that the titanium nitride is converted to titanium oxide by contact with the carbonate material. However, it has been found that this oxide has a larger volume and consequently locally pushes away the nickel layer. Moreover, it has been found that a titanium oxide layer obtained in this way is not impermeable and attack on the base material can consequently not be prevented.
In German Offenlegungsschrift 4030943 an anode made up of porous nickel and titanium oxide is described. On contact with lithium carbonate lithium titanate is produced, which promotes moistening of the porous nickel anode because carbonate material penetrates into the anode more easily. The separator plate or bipolar plate is nickel-coated and consists of stainless steel material.
The aim of the present invention is to provide a method for coating a separator plate of a fuel cell wherein a diffusion barrier layer is used which remains intact even after prolonged operation and wherein the corrosion-resistant nickel layer arranged thereon is not affected.
This aim is achieved with a method as described above in that said titanium compound comprises titanium oxide.
As a result of applying titanium oxide directly to the stainless steel base material, no harmful reaction takes place when carbonate diffuses through the porous nickel top layer. It is assumed that TiO2 in contact with the carbonate converts the top layer into Li2TiO3. This does not have any adverse effect on the electrical properties of the coating. The bulk of the TiO2 is converted into a sodium-titanium bronze or a potassium-titanium bronze. This material still has barrier properties relative to the alloy elements from the stainless steel. A further guarantee that the barrier layer remains intact even during a prolonged period is achieved by giving the layer an appreciable thickness. Preferably the thickness is at least 25 xcexcm and more particularly is between 40 and 50 xcexcm.
The titanium oxide layer described above can be applied in any way known from the prior art. Examples are application of a plasma spray under atmospheric pressure or high velocity oxygen flame spraying, sputtering, vaporisation using an arc, adhesion and sputtering using an arc, ion plating or CVD.
According to a further advantageous embodiment of the invention the titanium oxide is applied to the stainless steel support only after an adhesion layer, such as a layer of NiCrAlY, has been placed thereon.
As described above, the invention is used in particular for a fuel cell in which the separator plate or bipolar plate on the anode side has been treated as described above. It must be understood that the method described above can be used in other applications under aggressive conditions where electrical conduction is required.